Thanks to @David who suggested I look at the generated HTML rather than the python that builds it. When I looked at the HTML, I noticed my new page was missing a </form> at the bottom, though the python code had it.

I had put in a print('</form>') in a block near the beginning, which was incorrect, so the form was closed before the bottom part of my page was written. The python had written a </form> at the bottom too, but it was ignored since the form was already closed. Since much of the HTML was written after the form was closed, nothing in that part worked.

Making sure that there was a single </form> at the bottom of the page allowed the button to work and for me to continue to develop my change.